Background: Penicillin G and V have the same betalactam ring. Penicillin V (phenoxymethylpenicillin) results
from the substitution of the phenyl acetic acid of benzylpenicillin by the phenoxy methyl side chain. Methods: Our patient
was a 34-year-old man who experienced generalized urticaria after ingestion of Penicillin V. We performed skin prick
tests and intradermal tests with a battery of betalactams including Penicillin V. We also determined specific IgE against
penicillin V, penicillin G, amoxicillin, and ampicillin and performed a single-blind oral challenge with Penicillin V,
amoxicillin, cefuroxime, and ceftazidime. Results: The results of skin prick and intradermal tests with the betalactams included
were negative. Specific IgE with betalactams was < 0.10 IU/L. The result of a single-blind oral challenge with
Penicillin V was positive: 40 minutes after receiving 125mg of Penicillin V, the patient presented generalized pruritus
with hives on his back and chest. He tolerated oral administration of amoxicillin, cefuroxime, and ceftazidime. Conclusion:
We report an exceptional case of sensitization to Penicillin V with negative results in the allergy workup. Diagnosis
was based on a positive single-blind oral challenge result. The patient tolerated other betalactams. We provide a brief
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